1. Field of the Invention
This invention relates to a phase comparator and more particularly to a phase comparator for use in a servo system to maintain the speed of revolution and phase of rotary apparatus, such as a video disk player, to be constant.
2. Description of the Related Art
A conventional phase comparator has a circuit configuration as shown in FIG. 1. A reference signal from the reference oscillator 1 is converted by means of a monostable multivibrator (MMV) 2 into a pulse train signal (A) having a fixed pulse width. Switch 4 which is on/off controlled by this pulse signal discharges the electric charge in capacitor 3. This capacitor is charged by the electric current source 5 while switch 4 is off. The resulting voltage (B) at the terminal of capacitor 3 is applied to sample switch 7 through buffer 6. This sample output is held by capacitor 8 whose hold output (D) represents a phase error signal.
An RF signal of a signal to be regenerated, such as in a video disk player or the like from which a phase error signal is to be detected, is demodulated by the demodulator 9 and supplied to the synchronization signal separator 10. With the separated synchronization signal applied to the sample pulse generator 11, the sample pulse (C) is generated to control the switch 7.
FIG. 2 shows a series of waveforms illustrating the operation of the circuit shown in FIG. 1, in which waveforms (A) through (D) correspond to the waveforms of signals (A) through (D) discussed in connection with FIG. 1. Since the phase of the sample pulse changes in response to changes in the phase, if any, of the video signal to be regenerated, the sample point of the sawtooth signal changes causing the error signal level to vary in response to the sample point change.
In the circuit configuration, as described above, a normal error signal may be generated if the frequency of the reference signal is approximately equal to that of the regenerative synchronizing signal. If there is a large difference between the two frequencies, however, an accurate error signal cannot be generated due to the reason described below.
The sample pulse-to-pulse interval gradually increases as the frequency of the signal to be regenerated decreases. For example, the level of the error signal (D) increases from zero to the positive side as shown in FIG. 3 until it reaches the peak of the slope. Then, it reverses to the negative side. The level goes up again allowing the sampling operation to be performed from the lowest part of the slope. Accordingly, a signal having alternating positive and negative levels, as shown in FIG. 3, is produced as a phase error signal. The larger the frequency difference is, the smaller the positive and negative repetition cycle becomes. For the above reason, in the situation of controlling the servo rotary system of a signal regenerating apparatus, such as a video disk player, rotation control will be made difficult and the synchronization operation of the servo system will be disabled.
Referring now to FIG. 4, a frequency detector 30, in addition to the phase error detector 20 such as that described in connection with FIG. 1, is used. In such a system, the output signals from these two detectors are added by adder 31 whose output is used as a servo signal. With the inclusion of the frequency detector 30, the average value of the phase comparator output is zero although the phase comparator output alternately changes its polarity between the positive and negative values. Therefore the output of the frequency comparator 30 is used to control the rotary system to be synchronized with the synchronizing speed; then, the output of the phase comparator is applied. When the difference between the rotary system and the synchronizing speeds becomes small, the servo operation is initiated.
There is a disadvantage with this method, however, because two detecting means are necessary and complicated adjustment for accurate servo operation is required due to the mutual interference between the offsets of the two detecting means.
A potential problem arises which may be caused by phase shift at the time of the servo lock when two systems of detecting circuits, as shown in a conventional embodiment in FIG. 4, are used and there is a difference between the usual stability points.